For example, if a vertebra is diseased or damaged, it may have to be removed from the spine. In place of that vertebra a spacer implant is grafted in. Implants usually encompass mutually shiftable components as well as two special end plates serving to anchor the implant in the adjoining healthy vertebrae. In many other cases an internal fixation system is attached in a manner whereby it connects two vertebrae, with the fixation system bridging one or even several defective vertebrae. In the case of these predominantly plate-shaped implants the end-plate sections are fastened to the vertebrae by means of bone screws. The connecting elements between the end-plate sections are of a telescoping design, allowing the vertebrae next to the defective vertebral bodies to move parallel to the spinal axis in expansile or contractile fashion. Yet other spinal fixation systems consist of rod-shaped longitudinal supports that can be attached to pedicle screws via connecting elements. These connecting elements are so designed that they can slide along the longitudinal supports in an axial direction, again permitting expansile and contractile movement between the vertebrae. These provisions often require a distracting or compression of vertebrae or implant segments by means of appropriate instruments.
A device for distracting vertebrae has been described in U.S. Pat. No. 4,898,161 by GRUNDEI. This earlier distracting device features a pincer-like design of two levers axially extending into two jaws that can be equipped with pins. The jaws include special guide yokes and move in a parallel direction to each other when the pincers are opened or closed. By means of an adjustable bolt-and-nut joint between the two levers the pincer-like distracting device can be locked in position. A shortcoming of that earlier distracting device lies in its inability to produce any compressive displacement.
This invention is designed to remedy that problem. Adavantageously, this invention provides a device capable of distracting and compressing bone fragments, vertebral bodies or implant sections while at the same time permitting a very precise fine adjustment of the jaw spacing.
The invention achieves this by means of a device for applying axial forces on at least two bone fragments or vertebrae to which the device is connected.